1. Field of the Invention
The present invention relates to an electro-hydraulic forming tool and a method of forming a sheet metal blank in an electro-hydraulic forming operation.
2. Background Art
Electro-hydraulic forming tools and processes produce a shockwave by creating a high voltage discharge in a liquid that is in contact with the sheet metal blank to be formed. The shockwave in the liquid is propagated towards the blank and causes the blank to be deformed into an open die that has a forming surface. The shockwave forces the blank into engagement with the forming surface to form the metal blank into the desired shape.
Prior art electro-hydraulic forming tools have a liquid containing vessel that has an opening on its lower end. A blank holder is provided to hold a blank into engagement with the lower end of the vessel. By providing the vessel on top of the blank, air bubbles do not tend to form on the blank that could interfere with the forming process. Any air bubbles on the surface of the blank at the time the shockwave is propagated may cause surface defects in the formed part.
One disadvantage of electro-hydraulic forming is that each discharge of the electrodes to create the high voltage discharge in the fluid results in the formation of impurities in the water that results in vaporization of the electrodes. The impurities in the fluid may be projected by the shockwave toward the blank and may create surface defects in the surface of the part formed in the process.
Another disadvantage of conventional electro-hydraulic forming tools is that the fluid in the vessel generally must be drained and replaced for each tool cycle. The volume of fluid in the vessel for a larger part tends to be fairly substantial and a considerable portion of the cycle time of the tool is dedicated to draining and refilling the vessel.
Prior art electro-hydraulic forming tools generally have only one pair of electrodes that are energized to allow forming a part in a single step. Some prior art electro-hydraulic forming tools, such as that disclosed in U.S. Pat. No. 3,232,086 to Inoue and U.S. Pat. No. 3,566,645 to Lemelson disclose the concept of providing multiple sets of contacts in a vessel of an electro-hydraulic forming tool. Both of these patents provide multiple electrodes in a single chamber of a vessel. Regardless of which electrode is energized, it is not possible to direct the forming force toward a particular part of the blank or otherwise provide a staged forming process.
In electro-hydraulic forming operations in which water contained in the vessel directly contacts the blank, the panel may flash rust if not immediately treated. Further, it is difficult to control the fluid in the vessel since the only closure member provided is the blank that is to be formed in the process.
In a bladder press, a membrane is provided that separates a blank from a liquid and undergoes the same deformation as the blank itself. The membrane is subject to considerable deformation that is substantially equal to the deformation of the blank that is formed. Substantial friction is created between the membrane of the bladder press and the blank.
The above problems and others are addressed by Applicants' invention as summarized below.